Current Research
The research in my laboratory focuses on comparative molecular and biochemical neuroendocrinology and molecular evolution of vertebrate brain, pituitary hormones and their respective receptors. These studies have been supported by the National Science Foundation, National Institutes of Health, Great Lakes Fishery Commission, Sea Grant Program and other collaborative projects. My students, collaborators and I have helped to establish the lamprey as a model organism for neuroendocrinological/neuroscientific studies. Lampreys are exciting to study since they are one of two living representatives of the oldest extant lineage of vertebrates. The lampreys and hagfish, members of the jawless fishes called agnathans, form basal groups in the vertebrate lineage intermediate to Amphioxius (protochordates) and gnathostomes (jawed vertebrates). The phylogenetic relationships between hagfish, lamprey and gnathaostomes are still unresolved. Paleontological analysis of extinct agnathans had suggested that lampreys were more closely related to gnathostomes than either group is to the hagfishes (Forey and Janvier, 1993; 1994). However, Janvier and his collaborators reversed their position based on analysis of the complete mitochondrial DNA suggesting now that lamprey and hagfish form a clade (Delarbre et al., 2002), which is also supported by mitochondrial DNA analysis (Suga et al., 1999). These authors further suggested that due to unique anatomical and physiological characters of hagfish and lamprey that these characteristics should be re-examined and that the functional significance of these characters may be the “weighting criterion” in assisting to resolve the relationships of hagfish and lamprey to jawed vertebrates. Thus, information on the evolution of vertebrate brain/pituitary hormones and their genes in lamprey and hagfish can contribute to the ongoing phylogenetic analysis that may help in resolving the phylogenetic relationships between hagfish, lamprey, and jawed vertebrates.
Schematic diagram of the hypothalamic-pituitary-gonadal axis in the control of reproduction in the sea lamprey (Sower, 2003). Sower, S.A. 2003. The Endocrinology of Reproduction in Lampreys and Applications for Male Lamprey Sterilization. J. Great Lakes Res. 29:50-65.
Our investigations have led to the rapid advancement of lamprey neuroendocrinology by the discovery of the primary structure of GnRH-I and –III, receptor(s) and their corresponding cDNAs in lampreys and subsequent immunological and physiological experiments. In addition, and with my collaborators, we have determined the primary structure and cDNAs/genes of several other neurohormones and most of the pituitary hormones.
Silver, M.R., H. Kawauchi, M. Nozaki and S.A. Sower. 2004. Cloning and analysis of the lamprey GnRH-III cDNA from eight species of lamprey representing the three families of Petromyzoniformes. Gen. Comp. Endocrinol. 139:85-94
Silver, M.R., N.V. Nucci, A.R. Root, K.L. Reed and S.A. Sower. 2005. Cloning and characterization of a functional type II GnRH receptor with a lengthy carboxy-terminal tail from an ancestral vertebrate, the sea lamprey. Endocrinology. 146:3351-61.
Our recent information in lampreys promises a better understanding and adds additional support for the lamprey as a useful model for the molecular evolution of hormones and receptors from the invertebrates to later evolved vertebrates. As an example, we published a paper in Endocrinology (Kawauchi et al., 2002) in which we used cDNA cloning and protein isolation to identify, for the first time, a homolog of the growth hormone/prolactin/somatolactin family from the pituitary of sea lamprey. This is the first study in which a member of the growth hormone/prolactin/ somatolactin family had been identified in an agnathan. In addition, GH appears to be the only member of this hormone family in the sea lamprey. Evidence suggests that GH is the ancestral hormone in the molecular evolution of the GH family and that the endocrine mechanism for growth stimulation was established at an early stage of vertebrate evolution. Our current findings on a vertebrate of ancient lineage indicate that GH is the ancestral hormone and a forerunner of the GH family and that its gene duplicated during the early evolution of gnathostomes to form PRL and/or SL.
Through our Sea Grant, USDA, and NSF funding, we had several projects on hagfish reproduction and endocrinology. Several recent papers have been published from these studies. The latest findings on reproductive endocrinology of Atlantic hagfish (Myxine glutinosa) and implications for fisheries management are summarized in Powell et al., 2005. In response to a major decline or collapse of the fisheries (groundfish and anadromous species) industry in the Northeast, species that were once considered alternative or underutilized have and are being identified that may be suitable for commercial harvest, one such example is the hagfish. Hagfish in recent years have been sought after as valuable fish not only for their flesh, but also their skin. Currently, there are no regulations governing the harvesting of hagfish along the East Coast. There has been little to no information of the life history of hagfish including growth rate, age determination, reproductive biology, life span, and larval size at hatching. Thus, the level at which a sustainable fisheries for this species can be maintained is unknown. In some parts of the world, hagfish stocks are being depleted due to overfishing. In order for fisheries management to manage its hagfish stocks and develop a sustainable commercial hagfish fishery, critical information is needed to assist in determining the optimal use of this valuable resource.
Key elements of the reproductive system have not been elucidated in hagfish. However, there is new evidence from our recent reproductive studies that Atlantic hagfish may have a seasonal reproductive cycle. These data include seasonal changes in gonadotropin-releasing hormone (GnRH), gonadal steroids, estradiol and progesterone, corresponding to gonadal reproductive stages along with the putative identity of a functional corpus luteum. This newly acquired data may provide important information to fisheries managers of the East Coast.
There are over 54 ongoing projects in my laboratory, which include students and collaborators. My laboratory uses molecular, biochemical, physiological, and developmental techniques including isolation of cloned genomic and cDNA sequences of brain and pituitary hormones from fish and primates, gene expression, protein isolation and purification, immunocytochemistry, in situ hybridization, developmental studies, physiological studies, radioimmunoassays, ELISAs and receptor binding assays. We are now collaborating with Dr. Vernon Reinhold of the NH BRIN Center Protemics Grant in identifying and isolating GnRH and GTH using peptidomics and 2D gel electrophoresis. During the past two years, my laboratory has collaborated on genomics and proteomics in our two centers: NH BRIN Center of Structural Biology (proteomics) and the Hubbard Center for Genome Studies (genomics). In the emerging field of "Systems Biology", we are studying the expression of genes and proteins in the brain and pituitary in response to perturbations (i.e. endocrine disruptors) in order to study the complex biological system of the neuroendocrine control of reproduction.